Your main issue is length. The 19 will NOT work in the 17 and definitely not in the 34. I think you can squeeze the 17 into the 19 but due to some back issues I am laid up right now and cannot go check. Pretty sure it isn't so much longer as to not work though

The G34, G17L and G17 have the same locking lugs, and the G19 and G26 have the same locking lugs. This means that you can use a G34 or G17L barrel in a G17 but not the other way around (you can't use a G17 barrel in a G34 or G17L). You can also use the G19 barrel in a G26. You cannot however use a G17, G17L, or G34 barrel in a G19 or a G26.

Here you can see the barrel lengths: G26 G19 G17 G34 G17L (from left to right)

Here's a pic of various 9mm Glocks and how they compare in size and length.

"While the anti-gunners seem very concerned about the "one life" that your firearm might take -- they are not very concerned about the lives it will save." Jon H. Gutmacher, Florida Firearms - Law, Use & Ownership

1) So if you put a Glock 34 barrel in a Glock 17 you can have the end threaded to allow a suppressor?

I suspect the 34 barrel would fit in a Glock 17 because a Glock 34 is basically a 17 frame with a longer slide and barrel... However...

If I was going to put a suppressor on a Glock 17, I wouldn't buy a Glock 34 barrel and have it threaded... I'd get a new KKM barrel that will come threaded for the suppressor you want... for probably less money...

"While the anti-gunners seem very concerned about the "one life" that your firearm might take -- they are not very concerned about the lives it will save." Jon H. Gutmacher, Florida Firearms - Law, Use & Ownership